L. Robin M. Cocks

Earth geography from 400 to 250 Ma: a palaeomagnetic, faunal and facies review

Palaeomagnetic and faunal data have been re-evaluated on a global basis for the period from 400 Ma (early mid-Devonian) to 250 Ma (latest Permian). The boundaries of the major terranes are considered and defined. Six new palaeogeographical maps at 30 Ma intervals, which ensure kinematic continuity, are presented for this period. The palaeomagnetic data are very useful for positioning terranes for the present-day North Atlantic area, of variable value for China and Tarim, and for much of the large superterrane of Gondwana (being notably poor during the Early Devonian and the Early Carboniferous), and are sparse or non-existent for much of the rest of Asia. The relative positions of Laurussia and Gondwana at the end of the Palaeozoic when they united to form Pangaea are discussed, and it is concluded that the most convincing reconstruction (Pangaea A) is obtained by assuming an octupole contribution of 10–15% in combination with the main dipole of the Earths magnetic field. As well as faunal and palaeomagnetic data, the disposition of the major sediment types, including coal deposits, evaporites and glacial deposits, has also been considered, especially in the late Carboniferous and Permian.

Palaeontological evidence bearing on global Ordovician-Silurian continental reconstructions

Abstract The discreteness or otherwise of major Ordovician and Silurian terranes can be recognised by the shallow-water benthic faunas which lived upon them. Their borders are often indicated by the disposition of progressively shallow- to deep-water assemblages at the terrane edge as well as by structural features. Their positions relative to each other in the Early Palaeozoic can be best indicated by a combination of palaeomagnetic and faunal evaluation: the latter is the topic of this paper. Faunal evaluation is now possible quantitatively as well as quantitatively. Global palaeobiogeography is reviewed for the period as deduced from faunal evidence. There was one supercontinent, Gondwana, which stretched from West Gondwana (todays southern Europe and North Africa) at high latitudes to tropical East Gondwana (Australasia and adjacent areas), with intermediate palaeolatitudes in the Middle East and South America. Around Gondwana, especially to its north, were a large number of peri-Gondwanan terranes, particularly Avalonia, Perunica, parts of Turkey and Arabia and Sibumasu. In addition, there were the substantial independent continents of Laurentia, Baltica, Siberia, Annamia, North China and South China. Analysis of the shallow-water benthos, particularly trilobites and brachiopods, provides distinctive signatures for palaeo-position in most cases. Despite a large faunal turnover particularly corresponding with the latest Ordovician glacial event, the progressive evolution of the ecologies of benthic shelly faunas were also much influenced by changing geographies during the 80-Ma period. In the early Ordovician, oceans were at their widest, enabling Baltica and Laurentia to have different signatures from either East or West Gondwana. Siberia in early Ordovician times had faunal contact with Laurentia and East Gondwana, but in the mid-Ordovician, there were more endemics, and by the late Silurian, it was the only continent of substance in the northern hemisphere (hosting the Tuvaella Fauna). South China has varied faunal links but seems best treated as at the edge of the peri-Gondwanan collage for most of the period. We show how faunas document the early Ordovician rift of Avalonia from West Gondwana and its movement and subsequent collisions, first with Baltica in the end Ordovician and then with Laurentia in the early Silurian. Faunas also support the postulated movement of the Precordillera of South America from Laurentia in the early Ordovician to intermediate- to high-latitude Gondwana in the Silurian. We examine peripheral terranes bordering Iapetus to demonstrate their pre-collision positions. Analysis of some of the many terranes now forming Kazakhstan and adjacent areas in central Asia today reveals that the benthic faunas there have more affinity with Gondwanan and peri-Gondwanan faunas than with Baltica or Siberia, and thereby challenge structural models postulating an Early Palaeozoic Kipchak arc.

Chapter 2 New global palaeogeographical reconstructions for the Early Palaeozoic and their generation

Abstract New palaeogeographical reconstructions are presented at 10 myr intervals from the Lower Cambrian at 540 Ma to the Lower Devonian at 400 Ma, showing continental crustal fragments and oceans (not lands and seas), with appropriate kinematic continuity between successive maps. The maps were chiefly generated by revised and selected palaeomagnetic data and revised Apparent Polar Wandering paths linked to present-day polygons from the main continents. These have been reinforced by analysis of the distributions of some fossils and sediments. Gondwana was the dominating supercontinent from its final assembly in the Latest Neoproterozoic at about 550 Ma until the Carboniferous, and covered much of the Southern Hemisphere. The Northern Hemisphere was largely occupied by the vast Panthalassic Ocean. The relative positions of the major continents and the latitudes and rotation histories of Gondwana, Baltica, Siberia and Laurentia (Laurussia from the mid-Silurian) are now well known. Although Laurentia was oriented in a similar direction to the present, Siberia was inverted throughout the Lower Palaeozoic, and Baltica too was initially inverted, but rotated through 120° between the Late Cambrian and Late Ordovician before collision with Laurentia in the mid-Silurian Caledonide Orogeny. Through reconstructions of the Caledonide and some other orogenies, the progressive history of the Iapetus Ocean between Laurentia and Baltica/Gondwana is well constrained. Less major continents whose positions are also well known include Avalonia (initially peri-Gondwanan but migrating in the Early Ordovician to join Baltica by the end of the Ordovician), Sibumasu (now considered an integral part of Gondwana) and Mongolia (adjacent to Siberia). A large number of other terranes are reviewed and plotted on the reconstructions with varying degrees of certainty. However, significant continents with less well constrained or controversial positions are South China, North China (Sinokorea), Annamia (Indochina) and Arctic Alaska–Chukotka. The European areas of France, Iberia and southern Italy, previously considered by some as a separate Armorican Terrane Assemblage, remained parts of core Gondwana until the opening of the Palaeotethys Ocean near the end of the Silurian, but it is uncertain whether Perunica (Bohemia) was one of that group or whether it left Gondwana during the Middle Ordovician.

The Lower Palaeozoic palaeogeographical evolution of the northeastern and eastern peri-Gondwanan margin from Turkey to New Zealand

Abstract In Lower Palaeozoic times, Gondwana was by far the largest tectonic entity, stretching from the South Pole to north of the Equator, and is termed a superterrane. We consider the northeastern sector of the Gondwanan and peri-Gondwanan margin, from Turkey through the Middle East, the north of the Indian subcontinent, southern China and SE Asia, to Australia and New Zealand. There was progressive tectonic activity along some of its margins during the period, with areas such as southeastern Australia undergoing enlargement through the accretion of island arcs as that part of Gondwana rotated. However, most of the area, from the Taurides of Turkey to at least east of India, represented a passive margin for the whole of the Lower Palaeozoic. Other adjacent areas, such as the Pontides of Turkey and Annamia (Indochina), were separate from the main Gondwanan craton as independent terranes. The quality and quantity of available data on Lower Palaeozoic rocks and faunas varies enormously over different parts of this substantial area, and there are few or no detailed palaeomagnetic data available for most of it. Some workers have considered the string of terranes from Armorica to the Malaysia Peninsula as having left Gondwana together in the late Cambrian as a Hun superterrane, leaving a widening Palaeotethys Ocean between it and Gondwana. However, we consider that the Palaeotethys opened no earlier than in late Silurian time (with Armorica and other terranes to its north), and that the Hun superterrane was not a cohesive unity. Other researchers vary in presenting many substantial Central Asian and Far Eastern terranes, including North China, South China, Tarim, Annamia and others, as integral parts of core Gondwana and not leaving it until Devonian and later times. We conclude that North China, Tarim and Annamia, among others, were probably not attached to core Gondwana in the Lower Palaeozoic, that South China was close to Gondwana (but not an integral part of it), and that Sibumasu was probably part of Gondwana. We try to reconcile the very varied published geological data and opinions, and present new palaeogeographical maps for that sector of Gondwana and surrounding areas for the Cambrian (500 Ma), Ordovician (480 Ma) and Silurian (425 Ma).

Precisely locating the Ordovician equator in Laurentia

The Late Ordovician equatorial zone, like the zone today, had few hurricane-grade storms within 10o of the equator, as emphasized by the preservation of massive-bedded Thalassinoides ichnofacies in a trans-Laurentian belt more than 6000 km long, from the southwestern United States to North Greenland. That belt also includes nonamalgamated shell beds dominated by the brachiopod Proconchidium , which would not have been preserved after hurricane-grade storms. The belt lacks such storm-related sedimentary features as rip-up clasts, hummocky cross-stratification, or large channels. In contrast, other contemporaneous Laurentian Thalassinoides facies and shell beds on either side of the belt have been disturbed by severe storms below fair-weather wave base. The position of the biofacies-defined equatorial belt coincides with the Late Ordovician equator deduced from paleomagnetic data from Laurentia, thus providing both a high-precision equatorial location and an independent test of the geocentric axial dipole hypothesis for that time.

The Lower Palaeozoic margins of Baltica

Abstract The palaeocontinent of Baltica was palaeobiologically and geographically isolated in the early Ordovician, but drew closer to other palaeocontinents as the Ordovician progressed, colliding firstly with Avalonia in the latest Ordovician and then Laurentia to form Laurussia in the middle to late Silurian. Key faunas, particularly from the Baltic craton, chart this early isolation and subsequent changes. The Lower Palaeozoic margins of Baltica vary; much of the western margin was subducted during the Caledonian orogeny, the northern margin is poorly known and much of the eastern margin is preserved but heavily tectonised in the Urals. The south and south-eastern margins are the best preserved, despite Caledonian, Hercynian and later tectonism; with craton-margin sequences developed in the central Baltic to Denmark, northern Germany and Poland.

The Palaeozoic palaeogeography of central Gondwana

Abstract Nine new palaeogeographical maps of central Gondwana are presented at intervals within the Palaeozoic from the Middle Cambrian at 510 Ma to the end of the Permian at 250 Ma. The area covered includes all of Africa, Madagascar, India and Arabia as well as adjacent regions, including parts of southern Europe, much of South America (including the Falkland Isles) and Antarctica. After final assembly in the Late Neoproterozoic the southern margin was largely passive throughout the Palaeozoic, apart from some local orogeny in the Cambrian in the final stages of the largely Neoproterozoic Pan-African Orogeny and during the Late Palaeozoic Gondwanide Orogeny. The northern peri-Gondwana margin was active during the Early Palaeozoic but the NW part became passive by the earliest Ordovician when the Rheic Ocean opened between Gondwana and Avalonia. This was eventually followed by the latest Silurian or Early Devonian opening of the Palaeotethys Ocean between Gondwana and Iberia, Armorica and associated terranes and, much later, the rifting and opening of the Neotethys Ocean near the close of the Permian. In the Late Carboniferous, Gondwana merged with Laurussia to form Pangea. That accretion took place outside the area to the NW, although the consequent orogenic activity extended to Morocco and Algeria. Most of the centre of Gondwana was land throughout the Palaeozoic but with extensive shelf seas over the craton margins, particularly the northern margin from the Cambrian to the Devonian on which the important north African and Arabian hydrocarbon source rocks were deposited in the Lower Silurian (with the chief reservoirs in the adjacent Upper Ordovician) and Upper Devonian. There were also substantial Upper Carboniferous and later non-marine lake basins in central and southern Africa in which the Karroo Supergroup was deposited. The South Pole was located within the area from the Early Palaeozoic to the Mid-Permian and central Gondwana was therefore greatly affected by two ice ages: the short but sharp Hirnantian glaciation at the end of the Ordovician and another lasting sporadically for more than 25 Ma during the later Carboniferous and Early Permian.

Quantifying paleogeography using biogeography: a test case for the Ordovician and Silurian of Avalonia based on brachiopods and trilobites

Abstract Despite substantial advances in plate tectonic modeling in the last three decades, the postulated position of terranes in the Paleozoic has seldom been validated by faunal data. Fewer studies still have attempted a quantitative approach to distance based on explicit data sets. As a test case, we examine the position of Avalonia in the Ordovician (Arenig, Llanvirn, early Caradoc, and Ashgill) to mid-Silurian (Wenlock) with respect to Laurentia, Baltica, and West Gondwana. Using synoptic lists of 623 trilobite genera and 622 brachiopod genera for these four plates, summarized as Venn diagrams, we have devised proportional indices of mean endemism (ME, normalized by individual plate faunas to eliminate area biogeographic effects) and complementarity (C) for objective paleobiogeographic comparisons. These can discriminate the relative position of Avalonia by assessing the optimal arrangement of inter-centroid distances (measured as great circles) between relevant pairs of continental masses. The proportional indices are used to estimate the “goodness-of-fit” of the faunal data to two widely used dynamic plate tectonic models for these time slices, those of Smith and Rush (1998) and Ross and Scotese (1997). Our faunal data are more consistent with the latter model, which we use to suggest relationships between faunal indices for the five time slices and new rescaled inter-centroid distances between all six plate pairs. We have examined linear and exponential models in relation to continental separation for these indices. For our generic data, the linear model fits distinctly better overall. The fits of indices generated by using independent trilobite and brachiopod lists are mostly similar to each other at each time slice and for a given plate, reflecting a common biogeographic signal; however, the indices vary across the time slices. Combining groups into the same matrix in a “total evidence” analysis performs better still as a measure of distance for mean endemism in the “Scotese” plate model. Four-plate mean endemism performs much better than complementarity as an indicator of pairwise distance for either plate model in the test case.

Correlation for the Lower Palaeozoic

The global correlation standards now agreed for the Ordovician and Silurian systems, and partly so for the Cambrian System, are summarized. Correlation of the international series and stages with the traditional and revised British series and stages is reviewed, as well as with those in North America (Laurentia).

Strophomenate brachiopods from the late Ordovician Boda Limestone of Sweden: Their systematics and implications for palaeogeography

Synopsis Brachiopods of the superfamilies Strophomenoidea, Plectambonitoidea and Chilidi‐opsioidea are revised and described from the late Ordovician (Middle Ashgill) carbonate mud mounds of the Boda Limestone of Sweden. The fauna is compared and contrasted with those from mud mounds of comparable age in England (the Keisley Limestone) and Ireland (the Kildare and Portrane Limestones). New strophomenoidean genera are Trondomena and Ungulomena (both Glyptomenidae), the latter within the new subfamily Ungulomeninae. A new subgenus is Leptaena (Ygdrasilomena) (Rafinesquinidae), and new strophomenoid species are Gunnarella magnifica, Holtedahlina suedica, Katastrophomena (Costistrophomena)? magna, Luhaia candelabra, Leptaena (Leptaena) bergstroemi, Leptaena (Ygdrasilomena) roomusoksi, Kiaeromena (Kiaeromena)? grandis, Trondomena bella, Ungulomena lindstroemi and Christiania dalarnensis; new plectambonitoidean species are Bimuria popovi, Leangella (Leangella) longae, Anoptambonites williamsi and Ptychoglyptus ingenuus. Ten more unnamed and probably new species and at least two more new genera are described under informal nomenclature (A to J). The Strophomenoidea, with at least 20 named and unnamed species in the Boda Limestone, are found to have been much more diverse and endemic than the Boda Plectambonitoidea and the Chilidiopsoidea: the latter two superfamilies include no definite endemic genera and those that occur are relatively cosmopolitan in the low latitudes in which the Baltica Terrane was situated in the late Ordovician. Some faunal links are established with the neighbouring terranes of Avalonia, Laurentia and Siberia, but, in contrast to the plectambonitoideans and chilidiopsoideans, the relative endemicity of the Boda Strophomenoidea is striking and that local radiation was probably caused by the mid‐Ashgill Boda Warming Event.